This invention relates to novel unitary, storable, heat-curable epoxy resin systems and to the novel co-curing agents incorporated therein. It also relates to the process for preparing these novel co-curing agents by reacting a polyepoxide with a N-substituted piperazine or N-substituted homopiperazine to form an adduct. It further relates to the process for heat-curing epoxy resin systems containing a known curing agent, such as dicyandiamide or a carboxylic acid anhydride, and the novel co-curing agent to produce insoluble, infusible cured resin products.
The curing agents employed in the formulation of epoxy resins have a significant influence on the physical and chemical properties of the cured resins and the shelf life and/or pot life (working life) of the composition. As used herein, the terms "shelf life" and "pot life" refer to the resin system's storage stability at room temperature and elevated temperatures, respectively. Typical curing agents include aliphatic and aromatic amines, carboxylic acid anhydrides, mercaptans, amine-boron trifluoride complexes, dicyandiamide and dihydrazides. Unitary systems (one package) are desirable because they eliminate the additional mixing step before using, avoid the possibility that the consumer may use the incorrect amount of curing agent, and further avoid the need for complex packaging. Many of the known curing agents have drawbacks that limit their use in unitary systems.
Previous attempts to provide unitary systems have involved the use of amines as latent curing agents, i.e. curing agents which will be unreactive with epoxy resins at room temperature but which will react rapidly at elevated temperatures. Aliphatic amines cure (harden) too rapidly. Aromatic amines prolong the shelf life; however they require the use of much higher temperatures and longer cure times to effect the cure. Dicyandiamide (often called Dicy), dihydrazides, and boron trifluoride-ethylamine complexes are latent curing agents which give a shelf life of at least six months at room temperature. Unfortunately these curing agents suffer from a slow cure at elevated temperatures. Dicy, the most desirable latent curing agent because it gives the best strength properties, has a shelf life of over 1 year, but it requires 1 hour to cure at 170.degree. C. and does not cure at all at 120.degree. C.
In order to increase the curing rate and lower the curing temperature, co-curing agents (accelerators) are often added to epoxy resin systems containing Dicy. These accelerators usually are derivatized amines which are stable at room temperature but which decompose at elevated temperatures to release free amines which catalyze the curing.
Typical known accelerators include substituted ureas and bisureas, tetraalkyl guanidines, and imidazole adducts with mono- and polyepoxides as well as with polyepoxides and phenolic-novolac resins. 1,1'-(4-Methyl-m-phenylene)-bis-(3,3-dimethylurea) (see U.S. Pat. No. 3,386,956), when used in epoxy systems containing 10 parts Dicy and 10 parts of the urea, required 90 minutes to cure at 107.degree. C. and had a shelf life of only 5 weeks. Tetramethyl guanidine (see U.S. Pat. No. 3,391,113), when used in epoxy systems containing 10 parts Dicy and 5 parts of the guanidine, required 10 minutes to cure at 110.degree. C.; however the shelf life at room temperature was short and adding 1 part (o- or p-chloro)-benzoic acid only increased the shelf life from 2 to 7 days. The 2-ethyl-4-methyl imidazole adduct with the glycidyl polyether of 2,2-bis-(4-hydroxyphenyl)propane (see U.S. Pat. No. 3,756,984), when used in epoxy systems containing 5 parts Dicy and 3 parts of the adduct, required 10 minutes to cure at 180.degree.-185.degree. C. and 45-60 minutes to cure at 130.degree. C. 3-(p-Chlorophenyl)-1,1-dimethylurea (see U.S. Pat. No. 3,894,113), when used in epoxy systems which were solid at room temperature and contained 24.5 parts of carboxylated acrylonitrilebutadiene elastomer (as a flexibilizer), 6.3 parts Dicy and 3.1 parts of the urea, required 60 minutes to cure at 125.degree. C. The 2-methyl imidazole adduct with the diglycidyl ether of 2,2 -bis-(4-hydroxyphenyl)propane and phenolic novolac resins (see U.S. Pat. No. 4,066,625), when used in epoxy systems containing 5 parts Dicy and 7 parts of the adduct, required 10 minutes to cure at 138.degree. C.
Carboxylic acid anhydrides generally perform satisfactorily in epoxy resin systems at lower temperatures than Dicy; however they react very slowly and are generally unsuitable for use when a rapid cure is desired. The addition of co-curing agents, such as benzyldimethylamine, tris(dimethyl-amino-methyl)phenol or boron trifluorde-amine complexes, accelerates the cure; however the systems lack storage stability and in most cases must be used within 24 hours after the addition of the co-curing agent (accelerator) or else the mixture begins to cure under normal storage conditions. Dicyandiamide, when used as a co-curing agent in an epoxy resin system containing trimellitic anhydride (see U.S. Pat. No. 3,400,098), had a shelf life of only 2-3 days at 27.degree. C. and a pot life of 1 day at 49.degree. C.; the curing cycle ranged from 120 minutes at 93.degree. C. to 15 minutes at 204.degree. C. Phosphonium halides, e.g. ethyl triphenyl phosphonium iodide, when used as co-curing agents for epoxy systems containing hexahydrophthalic anhydride (see U.S. Pat. No. 3,547,885) accelerated the cure; however the systems are only stable in the absence of the polycarboxylic acid anhydrides.
Dihydrazides generally perform satisfactorily at temperatures in the range of 150.degree.-175.degree. C., requiring from 15 minutes to 2 hours to cure; they have a good shelf life and pot life. The addition of co-curing agents (activators), even in catalytic amounts, tends to destroy the shelf life. Some activators have been found which do not destroy the shelf life; however they substantially shorten the pot life. One such activator consists of equal parts of lead octoate and the reaction product of zinc octoate and diethylaminopropylamine (see U.S. Pat. No. 2,847,395).
Hence, the search continues for an ideal unitary epoxy resin system which will be stable for several months at room temperature and which will cure rapidly to give high performance products.
Accordingly, it is an object of the present invention to provide novel co-curing agents which will accelerate the curing rate of unitary, storable epoxy resin systems containing a known curing agent, such as Dicy, a carboxylic acid anhydride, or a dihydrazide, and lengthen or not seriously impair the shelf life and/or pot life.
It is a further object to provide a process for preparing and using these novel co-curing agents in unitary, storable epoxy resin systems and for forming insoluble, infusible resin products therefrom by heat-curing at elevated temperatures.